In a 3GPP access network, to enable the communication of a user equipment (UE) or a terminal to the external world, a PDN (Packet Data Network) connection is established, and a PGW (PDN Gateway) is selected for the PDN connection. As seen in FIG. 1, the PGW for the UE is selected by a serving MME (Mobility Management Entity) and it remains the same for the lifetime of the PDN connection. The PGW allocates an IP address for the UE and acts as the IP point of presence, i.e. all traffic to and from the UE is tunneled to the selected PGW. Inter-working with legacy 2G and 3G networks is achieved by defining an interface (S4) for 2G/GPRS (General Packet Radio Service) traffic and control signaling between a SGSN (Serving GPRS Support Node) and a SGW (Serving Gateway), as well as a direct tunnel interface (S12) between a RNC (Radio Network Controller) and the TGW for UTRAN (Universal Terrestrial Radio Access Network) UP traffic.
In many cases, the path of the traffic over the PDN connection continues from the PGW to the operator's service network or to an ASBR (Autonomous System Border Router), i.e. one of the operator's border routers constituting a peering point with other carriers, and further into the Internet.
In SAE/LTE (System Architecture Evolution/Long Term Evolution) networks, user packets travel through a significant portion of the access network encapsulated in a GTP-U (GPRS Tunnelling Protocol—User Data Tunnelling) tunnel. The GTP header of the incoming packet, especially one of its fields, the TEID (Tunnel Endpoint Identifier) is used in combination with the tunnel endpoint's IP address to explicitly identify a bearer of the terminal.
In SAE/LTE, the SGW and the base station use the TEID in the received packet's GTP header to forward the packet to the appropriate bearer. Based on the incoming TEID, the SGW selects a GTP or PMIP (Proxy Mobile IP) based tunnel leading to the PGW. Based on the incoming TEID, the eNodeB selects the appropriate radio bearer (leading to a specific terminal).
There are two alternative tunnelling possibilities between the SGW and the PGW. One is GTP tunnelling similarly as between the RBS (Radio Base Station) and SGW, and the other is using PMIP with GRE (Generic Routing Encapsulation) tunnelling.
In 3G networks, GTP is used between the SGSN and the RNC as well as between the SGSN and GGSN (Gateway GPRS Support Node). Based on the incoming TEID, the SGSN selects a GTP tunnel leading to the GGSN. Based on the incoming TEID, the RNC selects the appropriate radio bearer leading to a specific terminal through an appropriate NodeB. Alternatively, in the direct tunnel solution, GTP tunnelling is used directly between the RNC and GGSN. In EPC (Evolved Packet Core) networks, the traffic from 2G/3G access and SGW is also transported in GTP tunnels.
This tunnel-based forwarding mechanism is needed because the IP address does not necessarily identify the terminal unambiguously. In many cases, the terminal is allocated a local address that is later translated to a globally unique address by the PGW, thus it may easily happen that two terminals attached to the same RBS have the same IP address.
Lately with the introduction of broadband mobile technologies and change of subscriber behaviour shifting towards more high volume media download, coping with increased traffic volume in a mobile RAN (Radio Access Network) has become a problem. One possibility to off-load the access is to apply caches that would serve some parts of the requested content such as media, web, etc. In this way, it is possible to achieve transport gain above the cache. Additional benefits of RAN caching is the possibility for QoS/QoE enhancement by reducing transport delays and also reduced peering cost.
One relevant activity is the offload architectures for selected traffic such as the Internet traffic towards a defined IP network close to the terminal's point of attachment to the access network. A motivation for this requirement, together with the similar requirement for local IP access (LIPA) for the home (e)NodeB subsystem, is to decrease operator expenses, because in many cases it would be suboptimal to carry the traffic all way up to a central PGW, especially if that traffic may be offloaded locally at lower costs to a local service network or to the Internet.
Technical solutions for achieving SIPTO (Selected IP Traffic Offload) have been discussed and introduced into 3GPP standards. For Release 10, it has been decided that two main alternatives will be supported. One is based on selection of a closer PGW for specific types of traffic, and another is based on traffic breakout from the GTP tunnels on the Tu interface.
The advantage of the first conventional solution is that the traffic break-out, including break-out for caching purposes, may be achieved by architectural solutions that do not require significant changes to the current 3GPP architecture. However, it is questionable that designing small nodes with PGW functionality is economically feasible. Also, it may be problematic to provide secure network environment for these distributed GWs, whose functions (legal intercept, charging, policy enforcement) require such an environment.
The second conventional solution generally involve a type of a NAT (Network Address Translator) to be able to offload traffic from a tunnel below the IP point of presence. A similar solution is based on an enhanced NATting solution involving encapsulation and decapsulation based on the information stored in an extended NAT mapping table. However, this conventional solution does not deal with a number of related issues such as getting the tunnel information during attach and mobility. Addressing these issues would require a number of additional proprietary functions in the system, including intercepting mobility signalling messages, or interfacing to certain nodes to get required tunnel information.